Such nozzles, which are usually denoted two-fluid nozzles (TFN), are i.a. used for atomisation of a liquid in spray drying plants and in fluid bed agglomeration. Liquids can be in the form of solutions, dispersion or pure substances.
In particular, two-fluid nozzles are used when atomising a fluid, where fine droplets is the objective or where additional atomisation energy in the form of atomising gas is required to break up a fluid into droplets.
The mixing of the liquid and the gas may take place either inside the nozzle itself, so-called internal mixing, or outside the nozzle outlet, so-called external mixing.
With external mixing TFN, the free expansion of the gas has the disadvantage of being partly lost to the surrounding instead of adding energy to break up the liquid.
Internal mixing TFN has the advantage, compared to external mixing TFN, to mix gas and liquid before the two fluids enter the surrounding atmosphere through the outlet.
Criterions for evaluating the performance of a two-fluid nozzle are: the mean droplet size, the span of the droplet size distribution and not least the specific gas consumption, meaning the amount of gas used to atomise a given amount of liquid, also called the gas-to-feed ratio.
Atomizing finer droplets with a certain two-fluid nozzle means in general higher specific gas consumption. The specific gas consumption varies with type and size of two-fluid nozzle. In general, ratios between 1 and 2 (two gas-rate units to one feed-rate unit) are used. Rate is mass pr. time. The gas may be air, nitrogen, carbon dioxide, or any other suitable gas.
The span expresses how wide the droplet size distribution is. Aiming at a specific droplet size, a narrow distribution is desired. A wide distribution of the droplet size is generally disadvantageous.
The span, evaluated as (d90−d10)/d50 is normally found in the range from 1 to 3 for the type of nozzle in question and depending on the feed rate.
The contact and mixing of gas and liquid is where TFN meet their restrictions.
External mixing TFN, where the gas typically mixes with the liquid after leaving the nozzle through a ring-shaped aperture, meets the limitation when the gap in the gas exit becomes so large that a larger part of the gas is lost into the surrounding atmosphere, instead of reacting with the liquid.
Atomization into fine droplets follows when the liquid is spread out as a film, acting with the atomisation gas at high relative velocity to form droplets.
Internal mixing nozzles give the possibility of an efficient liquid-gas reaction, but is limited in capacity by internal channeling and channel dimensions.
Internal parts in the nozzle, intended for improving the gas-liquid mixing, also disturb the flow, causing the span of the droplet size distribution to rise. Internal parts in general complicate handling, cleaning and causes wear.
Addition of atomisation gas influences the spray drying or spray cooling process, in general causing a delay in the reaction following the atomisation.
Examples of nozzles of the internal-mixing type are well known in the art.
U.S. Pat. No. 2,612,405 discloses a nozzle in which the gas is supplied in the axial direction of the nozzle. Inside the gas supply pipe a drying air pipe and a guiding device is provided. The guiding device imparts a tangential deflection of the gas. The liquid is supplied in a pipe extending radically outside the gas pipe.
In a commercially available nozzle the atomising gas is supplied tangentially in a separate pipe, which contributes to the radial dimensions of the nozzle. Furthermore, the mixing chamber of this prior nozzle comprises edges and obstructions resulting from structural conditions.
International published application No. WO 00/58014 discloses a sprayer in the form of a nozzle having a tangential gas inlet to the mixing chamber and lateral liquid inlets. This nozzle suffers from insufficient mixing due to the geometry of the nozzle.